477
FXUS62 KCHS 232328
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
728 PM EDT Mon Sep 23 2024

.SYNOPSIS...
High pressure will linger across the region until mid week. A
tropical cyclone is forecast to move generally northward across
the Gulf of Mexico, then track inland across portions of
Florida, Georgia, and possibly South Carolina.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Early this evening: Radar imagery shows that the diurnal
convection that developed across the Midlands and pushed into
Jenkins, Screven, and Allendale counties is definitely on a
diminishing trend. There could still be an isolated shower or
two across these inland areas for the next few hours as
mesoscale boundaries interact, but anything that develops will
be weak. Otherwise, the rest of the night should be quiet. Model
guidance suggests that low stratus could spread into portions of
the Charleston Tri-County right around sunrise, and we could
also see some patchy shallow fog around sunrise too. No
significant visibility reductions are expected though. Lows are
forecast to bottom out in the low 70s in most areas.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
On Tuesday, H5 ridge axis is expected to slide east across the
forecast area during the day. Forecast soundings indicate weak
lapse rates under the ridge, limiting the convective potential.
The forecast will generally remain dry, with SCHC PoPs across
portions of the CHS Tri-county. High temperatures should remain
around 90 degrees.

Wednesday, the forecast area will remain under the influence of
the H5 ridge centered over the western Atlantic. At the sfc,
high pressure centered over the Mid-Atlantic States is expected
to ridge south across the region. It is possible that a sea
breeze could trigger isolated showers during the afternoon.
However, at this time, the forecast will remain dry. High
temperatures should remain in the upper 80s.

Thursday: Details of the forecast will largely depend on the
intensity, placement, and timing of Potential Tropical Cyclone
(PTC) Nine sourced from the Gulf of Mexico. See latest NHC
Tropical Weather Outlook for latest information on PTC 9. Based
on the collaborated low position, the center of the system is
expected to reach N. FL by Thursday afternoon. The circulation
associated with the system will remain quite large. In fact,
Tropical Storm force winds could reach the GA waters by Thursday
afternoon. GFS and ECMWF indicates that the leading edge of a
rain band will lift north, pushing across SE GA in the morning,
north of the Savannah River in the afternoon. In addition,
onshore winds could lead to an increasing tidal departure, with
some coastal flooding possible with the high tide Thursday
afternoon. It is possible that some excessive rainfall issues
may develop with any heavy rainfall that coincides with high
tide. High temperatures will be limited to 80 to 85 degrees.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
Thursday night, PTC 9 will pass between a H5 ridge centered
over the western Atlantic and a closed H5 low over the
MississippI River Valley. PTC 9 should accelerate across FL and
GA Thursday night. The circulation associated with the system
will cover the entire forecast area Thursday night. Given the
expected wind fields and rising values of tropical storm force
wind probs, it is expected that tropical headlines will be
needed for portions of the area.

Tornadoes: GFS shows the circulation from the system drawing
instability sourced from the Gulf Stream across SE GA/SC
Thursday night. Forecast soundings show the CAPE across the
coastal counties will surge to 500-1000 J/kg with LI as low as
-5. Interestingly, EHI values along the coast peak between 1-3
units by 6Z Friday. Hodographs indicate a strongly sheared
environment, with 0-1 km SRH in excess of 500 m2/s2. This
environment appears very suitable to fast-tracked tornadoes,
both over land and water.

The system is forecast to begin to pull away from the region on
Friday. Storm total rainfall amounts from the system is
expected to generally range from 3-4 inches, with locally higher
amounts. Ongoing gusty winds combined with the wet ground may
result in tree and power line damage.

The forecast is even more uncertain for Friday night into the
weekend, it is possible that dry air could wrap around the
central CONUS closed low and bring drier conditions to the
region. Or, some guidance indicates that a strong CAD may
develop over the region, with rain, thick cloud cover and cool
NE winds. The forecast will keep scattered convection with day
to day highs in the 80s.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
VFR conditions are expected to prevail at KCHS, KJZI, and KSAV
through 00z Wednesday. There could be some shallow ground fog
right around sunrise, but restricted visibilities are not
expected. Also, model guidance suggests low stratus could
attempt to spread in from the north and possibly approach KCHS
but current thinking is that it will remain north of the
terminal.

Extended Aviation Outlook: VFR conditions expected to prevail
through Wednesday. Gusty conditions are possible at the
terminals during the end of the week. Potential for periodic
flight restrictions are increasing.

&&

.MARINE...
Tonight: Broad High pressure will prevail, yielding tranquil
marine conditions. Expect SE winds 5-10 kt this evening, backing
to the S by daybreak Tuesday. Seas will generally be 2 ft
within 20 nm and 3 ft across the GA waters beyond 20 nm.

Tuesday and Wednesday: Broad high pressure offshore will result
in southeast winds around 10 kt or less on Tuesday, increasing
around 15 kts on Wednesday. Seas will average 1-3 ft for the
nearshore waters out 20 NM and 3-4 ft for the outer GA waters
from 20-60 NM.

Wednesday night through Saturday, there will be an increase in
winds due to a potentially developing tropical system in the
Gulf of Mexico. Thursday morning, guidance indicates that gusts
may range between 25-30 kts with seas build to 5-6 ft within 20
NM and 8-9 ft across the outer GA waters. Tropical Storm
conditions expected late Thursday afternoon until Friday
morning. Seas are expect to peak between 10 to 13 ft within 20
NM and 14-15 ft over the outer GA waters Thursday night.

Conditions will gradually improve late Friday through Saturday.

High Surf and Rip Currents: Five foot breakers are possible at
the beaches Thursday night. Additionally, an elevated risk for
rip currents is likely Thursday and Friday with gusty winds,
large breakers, and long period (10 second) swells.

&&

.TIDES/COASTAL FLOODING...
Although the astronomical high tides are lowering, minor coastal
flooding is expected with the daytime high tide cycle along the
Charleston and Colleton County coasts Tuesday, and potentially
Wednesday.

Strengthening onshore winds could lead to an increasing tidal
departure, with some coastal flooding possible with the high
tide Thursday afternoon.

This threat is expected to continue through the end of the week
due to impacts from a tropical cyclone.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$

NEAR TERM...BSH
SHORT TERM...NED
LONG TERM...NED
AVIATION...BSH/NED
MARINE...NED